I started by running 3.2 grains of Titegroup behind a 105 bullet. Occasionally, but often enough to ruin a few matches, got primers backed out that didn't reseat during recoil. The advise I got is the advise I'll pass on to you. Start a 3.5 grains of Titegroup and go down if you don't have any primer problems. After following this advise, I ran Titegroup at 3.4 grains for over a decade without experiencing a backed out primer or failure of any, other kind.
I have seen other shooters experience primer "back-out" problems with Titegroup running at, or below 3.2 grains with a 105.
(Parenthetically, I've run WST in my Wild Bunch loads for over ten years and feel it is a much better powder than Titegroup for light loads. Unfortunately, I've never worked it up in the .38 special, so I have no idea where to start. It would be worth playing with it, though, as it is a wonderful powder -- clean, quiet, consistent, neither position nor temperature sensitive in my experience)